Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, composed of carbohydrate polymers comprising cellulose, hemicellulose, glucans and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily amenable to saccharification enzymes. The pretreatment mixture is then further hydrolyzed in the presence of a saccharification enzyme consortium to release oligosaccharides and/or monosaccharides in a hydrolysate. Saccharification enzymes used to produce fermentable sugars from pretreated biomass typically include one or more glycosidases, such as cellulose-hydrolyzing glycosidases, hemicellulose-hydrolyzing glycosidases, and starch-hydrolyzing glycosidases, as well as peptidases, lipases, ligninases and/or feruloyl esterases. Saccharification enzymes and methods for biomass treatment are reviewed in Lynd, L. R., et al. (Microbiol. Mol. Biol. Rev. (2002) 66:506-577).
In order for the saccharification product, the biomass hydrolysate, to be used in subsequent fermentation production in an economical manner, it should contain a high concentration of sugars. A sugar concentration that is above 14% is desired in hydrolysate used for fermentation to ethanol, to produce ethanol at an economically viable level. For most types of lignocellulosic biomass, this corresponds to using a biomass dry matter content above 20% in a saccharification process. High sugar yielding saccharification of biomass at this high biomass concentration in an economically feasible reactor system has been heretofore been difficult to achieve.
Thus, there remains a need for an economical process for saccharification of biomass which can be carried out using a high dry weight of biomass such that the yields are high and the resulting hydrolysate contains a high concentration of fermentable sugars. In order to accomplish said economies and results, the process must provide for sufficient temperature and pH control. Applicants have been able to develop such a process by manipulating the biomass in ways that sustain thorough mixing in a low cost traditional stirred tank reactor system.